We do not know of anyone getting an STI from toilet seats, toilet floors, or objects/ paper that touched toilets. You do not need to worry about getting an STI this way. STIs are spread directly from person to person during oral, anal or vaginal sex.
It sounds like you have been very worried about this. If an STI test will help you to relax about this situation, then go ahead and get one, but again, there is no STI risk here. It may be helpful to talk about to a supportive person, health professional or counsellor if you are finding that this worry is having effects on your quality of lfe.
Let us know if that answers your question,